TFT mask ROM and method for making same

ABSTRACT

There is provided a monolithic three dimensional TFT mask ROM array. The array includes a plurality of device levels. Each of the plurality of device levels contains a first set of enabled TFTs and a second set of partially or totally disabled TFTs.

This application is a continuation-in-part of U.S. application Ser. No.09/927,648, filed on Aug. 13, 2001, and is a continuation-in-part ofU.S. application Ser. No. 09/961,278 filed on Sep. 25, 2001, both ofwhich are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention is directed generally to semiconductor devices andmethods of fabrication and more particularly to a thin film transistor(TFT) mask ROM and method of fabrication.

BACKGROUND OF THE INVENTION

Thin film transistors (TFTs) are utilized in various devices, such as aliquid crystal displays, static random access memories and innonvolatile memories. U.S. Pat. Nos. 5,572,046 and 5,383,149 alsosuggest using TFTs in a mask read only memory (mask ROM or MROM).Conventional TFTs have a structure that is similar to conventional bulkmetal oxide semiconductor field effect transistors (MOSFETs), exceptthat TFTs are formed in a semiconductor layer that is located above aninsulating substrate, such as a glass substrate, or above asemiconductor substrate that is covered by an insulating layer.

The mask ROM derives its name from the fact that this nonvolatile memoryis programmed or written using a custom mask during fabrication. In aconventional mask ROM fabrication process, a semi-finished array oftransistors is covered by a photoresist layer. This photoresist layer ispatterned using the custom photolithography mask to expose the channelregions of a first set of transistors while covering the channel regionsof a second set of transistors of the array. The exposed channel regionsare then rendered non-conductive by implanting the exposed channelregions of a first conductivity type with ions of the same conductivitytype in order to raise the threshold voltage of the implantedtransistors. Thus, the mask ROM array contains a first set of enabledtransistors and a second set of disabled transistors.

Each bit of information in a mask ROM is stored by the presence orabsence of a data path from a word (access) line to a bit (sense) line.The data path is eliminated by ensuring that no operational circuitelement (i.e., transistor) joins a word and a bit line, such as byincreasing the threshold voltage of selected transistors by theselective ion implantation. When a word line of a mask ROM is activated,the presence of a signal on the bit line will mean that a “1” is stored,whereas the absence of a signal will indicate that the bit location isstoring a “o”, as discussed on pages 619-621 of S. Wolf, SiliconProcessing for the VLSI Era, Vol. 2, Lattice Press, 1990, incorporatedherein by reference. Mask ROM arrays may be implemented as NOR or NANDarrays, for example.

The state of the art mask ROM has an effective cell area of 4 F², whereF is the minimum feature size. This cell area is larger than desirable,and leads to a less than optimum cell density, which increases the costof the mask ROM.

BRIEF SUMMARY OF THE INVENTION

A preferred embodiment of the present invention provides a monolithicthree dimensional TFT mask ROM array, comprising a plurality of devicelevels, wherein each device level comprises a first set of enabled TFTsand a second set of partially or totally disabled TFTs.

Another preferred embodiment of the present invention provides a TFTmask ROM array, comprising (a) a first plurality of spaced apartconductor rails disposed at a first height above a substrate in a firstdirection and (b) a second plurality of spaced apart rail stacksdisposed at a second height in a second direction different from thefirst direction. Each rail stack includes (i) a first semiconductorlayer whose first surface is in contact with said first plurality ofspaced apart conductor rails, (ii) a conductive film, and (iii) a gateinsulating film disposed between a second surface of the firstsemiconductor layer and the conductive film. The TFTs are formed atintersections of two adjacent first rails and the second rail stack. TheTFTs comprise a first set of enabled TFTs and a second set of partiallyor totally disabled TFTs.

Another preferred embodiment of the present invention provides a maskROM array, comprising a first set of enabled transistors containing acharge storage region, and a second set of partially or totally disabledtransistors.

Another preferred embodiment of the present invention provides a methodof making monolithic three dimensional TFT mask ROM array, comprisingforming a first device level comprising a plurality of TFTs over asubstrate, forming a first mask over the first device level, andpartially or totally disabling a first set of TFTs in the first devicelevel. The method also comprises forming a second device levelcomprising a plurality of TFTs over the first device level, forming asecond mask over the second device level, and partially or totallydisabling a second set of TFTs in the second device level.

Another preferred embodiment of the present invention provides a methodof making a TFT mask ROM array, comprising forming a first plurality ofspaced apart conductor rails disposed at a first height above asubstrate in a first direction, forming a first semiconductor layerwhose first surface is in contact with said first plurality of spacedapart conductors and forming a first mask over the first semiconductorlayer. The method also comprises selectively etching unmasked portionsof the first semiconductor layer or selectively implanting ions intounmasked portions of the first semiconductor layer, forming a conductivefilm, and forming a gate insulating film disposed between a secondsurface of the first semiconductor layer and the conductive film. Themethod also comprises forming a second mask, and patterning the firstsemiconductor layer, the gate insulating film and the conductive film toform a second plurality of spaced apart rail stacks disposed at a secondheight in a second direction different from the first direction.

Another preferred embodiment of the present invention provides a methodof making a mask ROM array, comprising forming an array of transistorscontaining a charge storage region, forming a mask over the array, andselectively totally or partially disabling unmasked transistors.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1, 3 and 5A and 5B are side cross sectional views of arraysaccording to the preferred embodiments of the present invention.

FIG. 2 is a three dimensional view of an array according to a preferredembodiment of the present invention.

FIGS. 4A-D are side cross sectional view of a preferred method of makingthe array of FIG. 1.

FIGS. 6-8 are circuit schematics of an array according to a preferredembodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In a preferred embodiment of the present invention, mask ROM density isincreased and its cost is decreased by forming a the mask ROM as amonolithic three dimensional array of TFTs. By integrating the array inthree dimensions, the effective cell area of the array is reduced to 4F²/N, where F is the minimum feature size and N is the number of devicelevels in the array.

FIG. 1 illustrates a monolithic three dimensional TFT mask ROM array 1according to a preferred embodiment of the present invention. The arraycomprises a plurality of device levels 2 separated by interlevelinsulating layers 3. There may be two or more device levels, such asfour to eight device levels 2, separated by one or more interlevelinsulating layers 3, such as three to seven insulating layers 3. Thedevice levels 2 alternate with interlevel insulating layers 3. The array1 is preferably formed over a substrate 4.

Each device level 2 contains a first set of enabled TFTs 5 and a secondset of partially or totally disabled TFTs 6. In one preferred aspect ofthe invention, the term “totally disabled” means that a particular TFTis rendered permanently off for any input voltage. In another preferredaspect of the invention, the term “partially disabled” means either thata threshold voltage of a particular TFT has been increased such thatthis TFT is in the off state for a particular value of input voltagethat would be sufficient to turn an enabled TFT on and/or that a sensecurrent of this TFT is lower than that of an enabled TFT. However, apartially disabled TFT may be turned on by providing an input voltagehigher than the input voltage sufficient to turn the enabled TFT on.

In one preferred aspect of the invention, the second set of TFTs 6comprises only totally disabled TFTs. In another preferred aspect of theinvention, the second set of TFTs 6 contains both totally and partiallydisabled TFTs. Thus, a so-called “multilevel” mask ROM array 1 is formedif the array contains partially disabled TFTs. An example of aconventional multilevel mask ROM is provided in U.S. Pat. No. 5,668,029,incorporated herein by reference. In a multilevel mask ROM, multiplelevels of conduction current may be sensed when the ROM array isaccessed, since a different amount of conduction current is sensed frombit lines of the enabled and the partially disabled TFTs. Of course, thesecond set of TFTs 6 may only contain partially disabled TFTs ifdesired.

It should be noted the array 1 may contain optional additional devicelevels (not shown in FIG. 1) which contain only enabled TFTs 5 or onlydisabled TFTs 6. Furthermore, the optional additional device levels maycontain devices other than TFTs and be used for circuits other than themask ROM. For example, peripheral or driver circuits may be formed inthe substrate 4 or in one or more array 1 device levels 2.

A TFT may be disabled by any means that prevents or restricts a flow ofdata (i.e., current) between the word line and the bit line contactingthe TFT. For example, the TFTs of the second set 6 may be disabled byremoving at least a portion of channel regions of these TFTs using amask, such as a photoresist mask, to render these TFTs totallyinoperative. Alternatively, the TFTs of the second set 6 may be disabledby selectively doping channel regions of these TFTs using a mask toincrease the threshold voltage of these TFTs above a predeterminedamount to render said TFTs partially or totally inoperative.Furthermore, the TFTs of the second set 6 may be totally disabled byremoving (i.e., etching) unmasked contacts between a word line or a bitline and the TFTs. For example, a gate electrode, source or drainelectrodes and/or source or drain regions may be removed to disable theTFT.

For example, in one preferred embodiment of the present invention, atleast a portion of channel regions of a first subset 7 of the TFTs ofthe second set 6 have been selectively removed to render the firstsubset 7 of the TFTs totally disabled, as shown in FIG. 1. The channelregions of a second subset 8 of the TFTs of the second set 6 have beenselectively doped to increase a threshold voltage of said TFTs to rendersaid TFTs partially disabled to form a multilevel mask ROM, as shown inFIG. 1. The first 7 and the second 8 subsets are disabled usingdifferent masks.

In an alternative embodiment of the present invention, the first subsetof TFTs 7 may be rendered totally disabled by a first threshold voltageadjustment implant, while the second subset of TFTs 8 may be renderedpartially disabled by a lower concentration threshold voltage adjustmentimplant. Preferably, the first and the second implants are carried outat the same time through the same photoresist mask using the same doseof implanted ions. The photoresist mask fully covers the channel regionsof the enabled TFTs, partially covers the channel regions of thepartially disabled TFTs, and does not cover the channel regions of thefully disabled TFTs. The difference in mask coverage results in adifferent concentration of ions being implanted into the TFT channelregions.

The TFTs 5 and 6 may comprise any type of metal oxide semiconductor(MOS) TFTs, such as top gate co-planar TFTs, top gate staggered TFTs,bottom gate co-planar TFTs and/or bottom gate staggered TFTs. Inco-planar TFTs, the source/drain electrodes and the gate electrode areon the same side of the channel. In staggered TFTs, the source/drainelectrodes and the gate electrode are on the opposite sides of thechannel. The array 1 may comprise one type of TFTs or more than one typeof TFTs. Furthermore, TFTs may comprise bipolar TFTs, if desired.

In one preferred embodiment of the present invention, the mask ROM arrayis provided in a “rail stack” staggered TFT configuration. FIG. 2illustrates one preferred configuration of an array 10 of top gatestaggered rail stack TFTs. The array 10 includes a first plurality ofspaced apart conductor rails 11 disposed at a first height above asubstrate (not shown for clarity) in a first direction. The array alsoincludes a second plurality of spaced apart rail stacks 12 disposed at asecond height in a second direction different from the first direction.Each rail stack includes a first semiconductor layer 13 whose firstsurface 15 is in contact with the first plurality of spaced apartconductor rails 11. Each rail stack 12 also includes a conductive film17 and a gate insulating film 16 disposed between a second surface 18 ofthe first semiconductor layer 13 and the conductive film 17. Each TFTcell 19 is formed at the intersection of two adjacent first rails 11 andone second rail stack 12. The TFT 19 channel length is determined by thespace between the adjacent rails 11, and its channel width is determinedby the width of the second rail stack 12. The TFTs 19 comprise a firstset of enabled TFTs and a second set of partially or totally disabledTFTs, as described with respect to FIG. 1.

The first plurality of rails 11 preferably comprise heavily dopedpolysilicon layers 21 of a first conductivity type (i.e., N+ or P+) incontact with metal or a metal silicide layers 22. The metal or metalsilicide layers 22 are preferably formed buried within the rails 11, butmay also be formed on the sides and/or bottom of the rails 11. The metalmay comprise aluminum, copper, tungsten or titanium (including titaniumnitride). The metal silicide may comprise any silicide, such astitanium, tungsten, cobalt, platinum or nickel silicide.

The first semiconductor layer 13 preferably comprises a polysiliconlayer of a second conductivity type (i.e., P− or N−). However, anamorphous silicon layer may be used instead. The substrate may be asilicon or other semiconductor substrate with an overlying interlevelinsulating layer. Alternatively, the substrate may comprise aninsulating material, such as glass, quartz, ceramic or plastic. Theconductive film 17 preferably comprises a polysilicon layer 23 and ametal silicide layer 24. As used herein, a “film” may contain one ormore “layers”.

The gate insulating film 16 may comprise a silicon oxide or a siliconnitride layer or any other one or more insulating layers. In onepreferred embodiment, film 16 comprises a portion of a charge storageregion. Preferably, film 16 comprises a portion of a charge storageregion, when it is desired to form a mask ROM and an EEPROM(electrically erasable programmable ROM) together in the same array oftransistors, as will be described in more detail below with respect toFIGS. 6-9. The charge storage region may comprises a dielectric isolatedfloating gate (i.e., a polysilicon floating gate located between atunnel dielectric and a control gate dielectric), an ONO dielectric film(i.e., a stack of silicon oxide/silicon nitride or oxynitride/siliconoxide layers) or an insulating layer containing conductive nanocrystals.The ONO dielectric film comprises a tunnel oxide, a charge storageSi₃N_(4-x)O_(1.5x) layer, where x is 0 to 1, and a blocking oxide. Thetunnel oxide has a thickness of 1.5 nm to 7 nm, preferably 2.5 nm. Thecharge storage silicon nitride or silicon oxynitride(Si₃N_(4-x)O_(1.5x)) layer has a thickness of at least 4 nm, preferably4-15 nm, most preferably 5 nm. The blocking oxide layer has a thicknessof 3 nm to 9 nm, preferably 4.0 nm. The insulating layer containing theconductive nanocrystals may comprise silicon, tungsten or aluminumnanocrystals dispersed in a silicon oxide, nitride or oxynitrideinsulating layer.

The polysilicon layers 21 of the first rails 11 comprise at least aportion of the TFT 19 source and drain regions. The conductive film 17comprises a gate of the TFTs. Portions of the first semiconductor layer13 comprise TFT channel regions of a second conductivity type. Ifdesired, optional doped regions 25 of a first conductivity type may beoutdiffused from layers 21 into the first semiconductor layer 13. Thus,optional regions 25 and layers 21 comprise the TFT 19 source and drainregions, while the metal or metal silicide layers 22 comprise the sourceand drain electrodes. Alternatively, regions 25 may be omitted andlayers 21 may comprise the entire source and drain regions of the TFTs19. Furthermore, if desired, intrinsic or lightly doped polysiliconlayers of the first conductivity type may be formed in the first rails11 between the heavily doped layers 21 and the first semiconductor layer13 to form offset or low doped drain (LDD) layers.

Preferably, a first planarized insulating layer 26, such as siliconoxide, is located between the first rails 11, and a second planarizedinsulating layer 27, such as silicon oxide, is located between thesecond rail stacks 12 (layers 26 and 27 are shown by dashed lines inFIG. 2 for clarity). Alternatively, one or more of silicon oxynitride,silicon nitride, spin-on glass, BPSG, PSG or BSG may be used instead ofor in addition to silicon oxide. Layers 26 and 27 may be planarized bychemical mechanical polishing or by other planarization methods.

In FIG. 2, the first rails 11 are located below the second rail stacks12 to form an array 10 of top gate staggered TFTs (i.e., the gate 17 isformed above the channel 13 on the opposite side of the source and drainregions 21). Alternatively, the first rails 11 may be located above thesecond rail stacks 12 (i.e., layers 13, 16 and 17) to form an array 20of bottom gate staggered TFTs, as shown in FIG. 3. If desired, thearrays 10 or 20 may be formed in a monolithic three dimensional array.In this case, the array comprises a plurality of device levels 2A-2C,each containing an array 10 or 20, separated by interlevel insulatinglayers 3A-3D, similar to the array shown in FIG. 1. However, since therail stack arrays 10 or 20 have a high density, these arrays maycomprise only one device level, if desired.

The mask ROM arrays of the preferred embodiments described above may bemade by any method. A preferred method of making a monolithic threedimensional TFT mask ROM array includes the following steps shown inFIG. 4A-D. A first device level 2A comprising a plurality of TFT channelregions is provided over a substrate 4. The first device level 2Acontains a semiconductor layer 9A that will used to form TFT channelregions. Preferably, layer 9A is formed over an insulating layer 3A,which is formed over the substrate 4, as shown in FIG. 4A.

A first mask 31, such as a photoresist mask, is formed over thesemiconductor layer 9A. Mask 31 covers all of layer 9A where the channelregions of the first set of TFTs 5 will be formed, except region(s) 33,where the channel regions of the second set of TFTs 6 will be formed, asshown in FIG. 4A. A second set of TFTs 6 in the first device level 2A isthen totally or partially disabled by selective etching or ionimplanting the unmasked channel regions 33 of the second set of TFTs 6.For example, selective ion implanting is schematically shown in FIG. 4Aby arrows and the “xx” symbol.

The TFTs 5 and 6 of the array are then completed by forming a gateinsulating layer on the channels, gate electrodes on the gate insulatinglayer, source and drain regions, and any other required layers for afunctional TFT, as shown in FIG. 4B. Alternatively, the step of forminga mask 31 and the step of disabling the TFTs 6 by etching or ionimplantation may be carried out after the first and the second sets ofTFTs 5 and 6 have been completed. Thus, the ion implantation may becarried out through the gate electrode into the channel. Likewise, theselective etching may be used to remove the gate electrode in additionto or instead of removing the channel.

A first interlevel insulating layer 3B is then formed over the firstdevice level 2A, as shown in FIG. 4B. Layer 3B may comprise one or moreof silicon oxide, silicon oxynitride, silicon nitride, spin-on glass,BPSG, PSG, BSG or any other insulating layers. The first interlevelinsulating layer 3B is preferably planarized by chemical mechanicalpolishing or etchback.

A second device level 2B comprising a plurality of TFT channel regionsis formed over the first interlevel insulating layer 3B, as shown inFIG. 4C. The second device level 2B contains a semiconductor layer 9Bthat will used to form TFT channel regions. A second mask 35, such as aphotoresist mask, is formed over the semiconductor layer 9B, such thatit covers all of layer 9B where the channel regions of the first set ofTFTs 5 will be formed, except region(s) 37, where the channel regions ofthe second set of TFTs 6 will be formed, as shown in FIG. 4C.

A second set of TFTs 6 in the second device level 2B are then totally orpartially disabled by selective etching or ion implanting the unmaskedchannel regions 37 of the second set of TFTs 6. The TFTs 5 and 6 of thearray are then completed by forming a gate insulating layer on thechannels, gate electrodes on the gate insulating layer, source and drainregions, and any other required layers for a functional TFT, as shown inFIG. 4D. If desired, a second interlevel insulating layer 3C is formedover the second device level 2B.

Thus, a three dimensional monolithic array is formed. Of course, thearray may contain more than two device layers 2A, 2B, such as four toeight device layers. To form additional device layers, a plurality ofinterlevel insulating layers are formed. A plurality of device levelscomprising a plurality of TFTs are formed over the plurality ofinterlevel insulating layers (i.e., each interlevel insulating layer isformed between adjacent device layers). A mask is formed over each ofthe plurality of device levels, and a second set of TFTs in theplurality of device levels is partially or totally disabled.

The term “monolithic”, as used above, means that layers of each level ofthe array were directly deposited on the layers of each underlying levelof the array. Thus, a first array of transistors is formed in a firstsemiconductor layer. Then, an interlayer insulating layer is formed overthe first array of transistors. A second semiconductor layer is formedover the second interlayer insulating layer. A second array oftransistors is formed in the second semiconductor layer to form amonolithic three dimensional array. Additional array levels may beformed in the same fashion if desired. In contrast, two dimensionalarrays may be formed separately and then packaged together to form anon-monolithic memory device.

A similar method may be used to form an array of rail stack TFTs shownin FIG. 2 or 3. To form the array 10 of top gated TFTs shown in FIG. 2,a first plurality of spaced apart conductor rails 11 are formed at afirst height above a substrate in a first direction. Then, a firstsemiconductor layer 13 is formed such that its first surface 15 is incontact with said first plurality of spaced apart conductors 11.

A first mask 31 is formed over the first semiconductor layer 13, similarto that shown in FIG. 4A. The unmasked portions of layer 13 areselectively etched or ion implanted, similar to that shown in FIG. 4A.Thereafter, the gate insulating film 16 and conductive film 17 areformed over the etched or implanted first semiconductor layer 13. Then,a second photoresist mask (not shown) is formed over the conductive film17. The first semiconductor layer 13, the gate insulating film 16 andthe conductive film 17 are then patterned (i.e., etched) using thesecond mask to form a second plurality of spaced apart rail stacks 12.The rail stacks are disposed at a second height in a second directiondifferent from the first direction, as shown in FIG. 2.

To form the array 20 of bottom gated TFTs of FIG. 3, the order of stepsis reversed. The conductive film 17 and the gate insulating film 16 andthe first semiconductor film 13 are formed in this order. Then, thefirst mask 31 is formed over layer 13 and the selective etching or ionimplantation of layer 13 is performed. Then, the films 17 and 16 andlayer 13 are patterned to form the second rail stacks 12. If desired,the rail stacks 12 may be formed before forming the first mask 31 onlayer 13. Then, the first rails 11 are formed on the second rail stacks12, as shown in FIG. 3. If desired, the arrays 10 and 20 may be includedin a monolithic three dimensional array by forming an interlevelinsulating layer over each array and forming another array 10 or 20 overthe interlayer insulating layer, as many times as desired.

FIGS. 1, 2 and 4A-D illustrate a monolithic three dimensional arraywhere adjacent device levels are separated by an interlevel insulatinglayer. However, in another preferred embodiment of the presentinvention, some or all interlevel insulating layers are omitted and atleast two adjacent device levels in a monolithic three dimensional arraycontact each other, as shown in FIG. 5A. The exemplary array 30illustrated in FIG. 5A contains four device levels 2A-2D. Of course,there may be more or less than four device levels. The array 30 issimilar to arrays 10 or 20, except that layers of the array 30 arecopied in a mirror image fashion in the vertical direction. The layersin FIG. 5A are numbered in the same fashion as in FIGS. 2 and 3, exceptthat the layers have a suffix, such as “A” through “D”, depending onwhich device level 2A-D contains the particular device layer.

In the layout of FIG. 5A, rails 11A are shared between adjacent devicelevels 2A and 2B, while rails 11B are shared between adjacent devicelevels 2C and 2D. Likewise, the conductive layer (i.e., gate electrode)17B of rail stack 12B is shared between adjacent device levels 2B and2C. Thus, adjacent device level pairs 2A/2B, 2B/2C and 2C/2D contacteach other. However, each device level 2A-D contains its own channellayer 13A-D and its own gate insulating layer 16A-D, which are notshared with adjacent device levels. Thus, each rail stack that is sharedbetween device levels contains a conductive layer 17 between two gateinsulating layers 16 and two channel layers 13. Device levels 2A and 2Ccontain bottom gated TFTs, while device levels 2B and 2D contain topgated TFTs. The array 30 may contain n-type or p-type TFTs, as desired.The TFTs of array 30 comprise a first set of enabled TFTs and a secondset of partially or totally disabled TFTs, as described with respect toFIG. 1.

FIGS. 2, 3 and 5A illustrate an array of n-type TFTs (i.e., NMOS) orp-type TFTs (i.e., PMOS) according to the preferred embodiments of theinvention. FIG. 5B illustrates one device level of a vertically stacked,common gate CMOS mask ROM array in a rail stack configuration accordingto another preferred embodiment of the present invention. The CMOS arrayin FIG. 5B is similar to the arrays illustrated in FIGS. 2, 3 or 5Aexcept that transistors of different charge carrier type are formed oneither side of the gate line. In FIG. 5B, the NMOS transistors arearranged below the PMOS transistors. However, it should be understoodthat the PMOS transistors may be arranged below the NMOS transistors ifdesired.

In FIG. 5B, the array of CMOS devices 100 is preferably formed over aplanarized interlayer insulating layer 101, such as a CMP planarizedsilicon oxide layer. Layer 101 is formed over a substrate (not shown) asin the previous embodiments. Each CMOS device is thus a CMOS TFT becauseit is formed over an insulating layer. However, the CMOS devices may beformed in a monocrystalline silicon substrate, if desired.

The array includes a plurality of gate lines (i.e., word lines) 103(only one gate line is shown in the cross sectional view of FIG. 5B).Preferably the gate line comprises a first N+ polysilicon layer 105, asilicide layer 107, such as a TiSi_(x) or WSi_(x) layer, over the firstpolysilicon layer and a second P+ polysilicon layer 109 above thesilicide layer. The gate line 103 acts as a gate electrode in each TFT.Thus, no separate gate electrodes connected to the gate lines arerequired.

A first insulating layer 111 is disposed adjacent to a first side of thegate electrode 103. This insulating layer 111 may be a conventional gatedielectric. If it is desired to form a mask ROM and an EEPROM in thesame array of transistors, then the insulating layer 111 is preferably acharge storage layer, such as an ONO stack or isolated nanocrystals, toform charge storage CMOS TFTs, such as EEPROM CMOS TFTs. If floatinggate type EEPROM CMOS TFTs are desired, then a floating gate and acontrol gate dielectric may be added between the insulating layer 111and the gate line 103.

A p-type semiconductor layer 113, such as a P− polysilicon layer, isdisposed on a side of the first insulating layer opposite to the gate103. This layer contains the NMOS TFT bodies. N+ source and drainregions 115 are disposed in layer 113. The portions of layer 113 betweenregions 115 comprise NMOS TFT channel regions.

Preferably, the source and drain regions 115 are formed by outdiffusionof n-type dopants from the source and drain electrodes (i.e., bit lines)117. However, regions 115 may be formed by any other method, such as bymasking and ion implantation or may be omitted entirely. The electrodes117 contact the source and drain regions 115 and are disposed on thebottom of the p-type semiconductor layer 113 (i.e., on the side of layer113 opposite to the first insulating layer 111). Preferably, theelectrodes 117 comprise N+ polysilicon rails which extend in a directionperpendicular to the gate line 103. If desired, an optional metal ormetal silicide layer is formed in contact with electrodes 117 toincrease their conductivity. However, the electrodes 117 may comprisemetal or metal silicide instead of the heavily doped polysilicon, ifdesired. A planar insulating filler layer 118, such as silicon oxide, isdisposed between the source and drain electrodes 117.

Thus, each NMOS TFT 119 is located between adjacent source and drainregions 115 and comprises a portion of layers 105, 111, 113 and 117, asillustrated in FIG. 5B. The PMOS TFTs 121 are located above the NMOSTFTs 119.

The PMOS TFTs 121 include a second insulating layer 123 adjacent to asecond side of the gate electrode 103. In FIG. 5B, layer 123 is locatedon the P+ polysilicon layer 109 of the gate line 103. The insulatinglayer 123 may be a conventional gate dielectric. If it is desired toform a mask ROM and an EEPROM in the same array of transistors, then theinsulating layer 123 is preferably a charge storage layer, such as anONO stack or isolated nanocrystals, to form charge storage CMOS TFTs,such as EEPROM CMOS TFTs. If floating gate type EEPROM CMOS TFTs aredesired, then a floating gate and a control gate dielectric may be addedbetween the insulating layer 123 and the gate line 103.

An n-type semiconductor layer 125, such as an N− polysilicon layer, isdisposed above the second insulating layer 123. Layer 125 is disposed onthe opposite side of layer 123 from the gate electrode 103. P+ sourceand drain regions 127 are disposed in layer 125, such that regions oflayer 125 between the source and drain regions 127 comprise channelregions of PMOS TFTs. Source and drain electrodes 129 are disposed overthe N− polysilicon layer 125 and in contact with the source and drainregions 129. Thus, the electrodes 129 are disposed on top side of the N−polysilicon layer 125 opposite to the second insulating layer 123. Aplanar insulating filler layer 131, such as silicon oxide, is disposedbetween the source and drain electrodes 129. If desired, an optionalmetal or metal silicide layer is formed in contact with electrodes 129to increase their conductivity.

Thus, each PMOS TFT 121 is located between adjacent source and drainregions 127 and comprises a portion of layers 109, 123, 125 and 129, asillustrated in FIG. 5B. The NMOS TFT 119 and the PMOS TFT 121 directlyabove it together comprise a TFT EEPROM CMOS device, which is an NMOStransistor and a PMOS transistor with a common gate electrode 103, andwith each preferably using a charge storage medium for a gatedielectric. If desired, the CMOS structure may be inverted and the PMOSTFTs formed below NMOS TFTs. It should be noted that NMOS and PMOSelectrodes (i.e., bit lines) do not have to fall directly on top of eachother, although they preferably should have the same pitch. NMOS andPMOS transistors thus can have different channel lengths, but the pitch(and thus array size) will be limited by the longer of the two channellengths. The TFT 119, 121 channel length is determined by the spacebetween the adjacent rails 117 or 129. The array shown in FIG. 5B may beextended to a monolithic three dimensional array by forming additionaldevice levels in the vertical direction with or without interlevelinsulating layers, as shown in FIGS. 3, 4A-D or 5A.

The TFTs 119 and 121 comprise a first set of enabled TFTs and a secondset of partially or totally disabled TFTs, as described with respect toFIG. 1. Thus, some NMOS TFTs are enabled while others are disabled usinga mask. Likewise, some PMOS TFTs are enabled while others are disabledusing a mask. In one preferred aspect, TFTs of one conductivity type(i.e., NMOS or PMOS TFTs) contain a charge storage layer or region,while TFTs of the other conductivity type (i.e., PMOS or NMOS) do nothave a charge storage region or layer. Thus, the CMOS of this aspectcomprises one EEPROM TFT and one non-EEPROM TFT. In this case the EEPROMis formed in the portion of the array containing charge storage TFTs.

As described above, the transistors in the mask ROM array preferablycontain a charge storage region. Thus, the transistors for the mask ROMmay also be used for an EEPROM. The transistors may comprise any type ofTFTs or metal oxide semiconductor field effect transistors in a bulksilicon substrate. The addition of a charge storage region totransistors of a mask ROM array is particularly advantageous because itallows some cells in the array to be preprogrammed prior to packagingthe array in the factory using the mask ROM techniques, while it allowsother cells in the array to be programmed in the field after packagingthe array using EEPROM programming techniques (such as hot carrierinjection or Fowler-Nordheim tunneling). The two stage programming ofthe array is advantageous because it improves the flexibility of thedevice processing. For example, the standard cells in the array may bepreprogrammed in the factory, while the user-desired cells may bepreprogrammed in the field by the user. However, since the mask ROM andthe EEPROM are formed in the same array of transistors containing acharge storage region, the manufacturing costs are decreased, sinceseparate mask ROM and EEPROM transistors do not have to be fabricated.

An example of two stage programming is illustrated using an staticrandom access memory (SRAM) preprogrammed by mask ROM techniques and alogic gate programmed in the field. FIG. 6 illustrates a circuitschematic of an unprogrammed TFT CMOS array 100 of FIG. 5B. The array100 is a matrix of NMOS 119 and PMOS 121 devices with common gates 103.In one portion of the array 100, a 5×6 SRAM cell 180 is formed by maskROM preprogramming, as described with reference to FIGS. 5 and 7. Theprogramming process comprises the following steps.

First, the rails 117 and the p-type semiconductor layer 113 are formed.Then, a first mask is formed over layer 113 of all NMOS TFTs 119 in thecircuit of FIG. 7, before the charge storage layer 111 is formed onlayer 113. The mask covers regions where NMOS access transistors 189 and190 and NMOS inverter transistors 191 and 192 will be formed. Thechannel portions of layer 113 in areas where the other NMOS transistorswill be formed is etched such the other NMOS TFTs 119, 150 in FIGS. 5and 7, respectively, are disabled.

Second, the first charge storage layer 111 is formed over the patternedlayer 113. A second mask is formed over layer 111. Layer 111 is removedat locations 148 in FIG. 7. The gate line 103 is then formed on thepatterned layer 111. Since layer 113 is removed at locations 148, thegate lines (103 in FIG. 5B, 181 and 183 in FIG. 7) are shorted to thesource and drain rails (117 in FIG. 5B, 185, 187 in FIG. 7) at locations148.

The second charge storage layer 123 is then formed on the gate line 103.A third mask is formed over layer 123. Layer 123 is removed at locations148 in FIG. 7.

The n-type semiconductor layer 125 is then formed over the patternedlayer 123. A fourth mask is formed over layer 125. The mask coversregions where PMOS inverter transistors 193 and 194 will be formed. Thechannel regions of layer 125 in areas where the other PMOS transistorswill be formed is etched such the other PMOS TFTs 121, 150 in FIGS. 5and 7, respectively, are disabled.

The rails 129 are formed over the patterned layer 125. Since layers 123and 125 are removed at locations 148 in FIG. 7, the gate lines (103 inFIG. 5B, 181 and 183 in FIG. 7) are shorted to the source and drainrails (129 in FIG. 5B, 186, 188 in FIG. 7) at locations 148.

Thus, a 5×6 SRAM cell 180 is formed, as shown in FIG. 7. In the SRAMcell, the transistors 189 and 190 are the SRAM access transistors, whiletransistors 191, 192, 193 and 194 are the cross coupled inverters. Thecell is accessed by placing a positive voltage on the word line 195.Data is input onto and read out of BL and BL-bar, which are providedinto bit lines 196 and 197, respectively. Voltages V_(SS) and V_(DD) areprovided into bit lines 198 and 199, respectively.

Since all of the “links” (gate/bitline shorts) and the “antilinks”(removed channels) are formed prior to field programming, all of thelinkable nodes do not have to be brought out of the array. This allows a6 transistor SRAM bit cell with an area of about 80 F² to be made.

After the SRAM 180 is preprogrammed, one or more logic devices, such asinverters, NAND gates and/or NOR gates are programmed in the field bythe end user. The logic devices may be programmed by EEPROM and/orantifuse programming techniques (i.e., where a link between the gate andsource or drain region is formed). As shown in FIG. 8, another portionof the unprogrammed array 100 is programmed to form a 4×4 inverter cell143. This portion may be laterally spaced from the SRAM 180 or it may belocated in another device level, if the array comprises a threedimensional array.

First, a high voltage is applied between gate (i.e., word) line 145 andbit lines 147, which will be used to carry the output voltage, V_(out).This causes conductive antifuse links 148 to form to electricallyconnect lines 145 and 147. Then, the driver circuit provides aprogramming voltage to all other transistors 150 to increase theirthreshold voltage to turn them off, except to NMOS transistors 155 andPMOS transistors 157. The NMOS 155 and PMOS 157 transistors form theinverter. When a high voltage, Vin, is provided into gate line 149, thena low voltage, V_(out), is read out, and vice-versa. Voltages V_(SS)(i.e., ground) and V_(DD) (i.e., power supply voltage) are provided intobit lines 151 and 153 which are connected to transistors 155 and 157.

Thus, the array contains a plurality of TFTs with charge storageregions, some of which are programmed by mask ROM techniques, others byEEPROM techniques, while yet others are programmed by anti-fusetechniques. Thus, the array programming flexibility is increased whileits cost is reduced because all the transistors in the array are thesame.

In the above described preferred embodiments, the use of polysiliconlayers was described. However, amorphous silicon, single crystal siliconor non-silicon semiconductor materials may be used instead. Thepolysilicon layers described above may be deposited as polysiliconlayers by chemical vapor deposition (CVD) or other methods. Thesemiconductor layers may be in-situ doped during deposition or theselayers may be doped by ion implantation or diffusion after deposition.Furthermore, the polysilicon layers, such as the channel layer, may befirst deposited as amorphous silicon layers and then crystallized toform polysilicon or single crystal silicon layers. The crystallizationmay be carried out by laser, flash lamp and/or thermal (i.e., furnace)annealing. If desired, a transition metal or germanium crystallizationcatalyst material may be placed in contact with the amorphous siliconlayers to increase the polysilicon grain size after the crystallization.Such transition catalyst materials may comprise nickel, cobalt,platinum, palladium and other transition metals. The insulating andconductive layers described above may be deposited by any known method,such as CVD, sputtering, plating, spin-on coating (for spin-on glass),etc.

In the various embodiments described above, metal silicide layers wereformed in contact with silicon layers, such as the polysilicon sourceand drain regions or gate electrodes. One preferred method of forming ametal silicide layer, such as a titanium silicide layer, in contact witha silicon layer is by using a silicon cap and a TiN layer. The titaniumsilicide layer is formed on an undoped amorphous silicon cap layer. Thecap layer is formed on a heavily doped silicon layer, such as apolysilicon or amorphous silicon layer doped to a concentration inexcess of 10¹⁹ cm⁻³, such as 10¹⁹ cm⁻³ to 10²¹ cm³. The cap layer ispreferably deposited on P+ polysilicon or N+ amorphous silicon layers.The N+ amorphous silicon may then be recrystallized into N+ polysiliconduring subsequent annealing steps.

A method of forming a titanium silicide (TiSi₂) layer comprises thefollowing steps. A heavily doped polysilicon layer is deposited. Forexample, a P+ polysilicon layer is boron doped to a concentration of5×10²⁰ cm⁻³, and has a thickness of about 1400 Angstroms. A cap layer ofundoped amorphous silicon is deposited on the P+ polysilicon layer. Thecap may be 600 Angstroms thick, for example. A titanium layer isdeposited on the cap. The titanium layer may be 250 Angstroms thick, forexample. A titanium nitride layer is deposited on the titanium layer.The titanium nitride layer may be 100 Angstroms thick, for example.Other layer thicknesses may be used, as required.

The layers are annealed at a temperature below 650° C. for less thanfive minutes to react the titanium and the silicon in the cap to form aC49 phase TiSi₂ layer. The anneal may be carried out at 600° C. for 1minute, for example. If desired, another P+ polysilicon layer isdeposited over the stack and the stack is etched into a thin “wire” or“rail”, such as a word line or bit line. The wire or rail may be 0.25microns wide or less. The titanium silicide is then transformed from theC49 to the C54 phase by a high temperature (i.e., above 650° C.) anneal.The anneal can take place before or after the wires or rails arepatterned, at 800° C. for one minute, for example. By annealing eachSi/Ti/TiN film stack below 650° C., dopant diffusion and thermalgrooving of the TiSi₂ is minimized. Multiple film stacks can bedeposited and etched sequentially.

The foregoing description of the invention has been presented forpurposes of illustration and description. It is not intended to beexhaustive or to limit the invention to the precise form disclosed, andmodifications and variations are possible in light of the aboveteachings or may be acquired from practice of the invention. Thedrawings and description were chosen in order to explain the principlesof the invention and its practical application. The drawings are notnecessarily to scale and illustrate the device in schematic blockformat. It is intended that the scope of the invention be defined by theclaims appended hereto, and their equivalents.

1. A TFT mask ROM array, comprising: a first plurality of spaced apartconductor rails disposed at a first height above a substrate in a firstdirection; a second plurality of spaced apart rail stacks disposed at asecond height in a second direction different from the first direction,each rail stack including: a first semiconductor layer whose firstsurface is in contact with said first plurality of spaced apartconductor rails; a conductive film; and a gate insulating film disposedbetween a second surface of the first semiconductor layer and theconductive film; wherein the TFTs are formed at intersections of twoadjacent first rails and the second rail stack; and wherein the TFTscomprise a first set of enabled TFTs and a second set of partially ortotally disabled TFTs.
 2. The array of claim 1, wherein: the firstplurality of rails comprise polysilicon layers of a first conductivitytype in contact with metal or a metal silicide layers; the firstsemiconductor layer comprises a polysilicon layer; the gate insulatinglayer comprises a portion of a charge storage region; and the conductivefilm comprises a polysilicon layer and a metal silicide layer.
 3. Thearray of claim 2, wherein: the polysilicon layers of the first railscomprise at least a portion of the TFT source and drain regions; theconductive film comprises a gate of the TFTs; portions of the firstsemiconductor layer comprise TFT channel regions of a secondconductivity type; and the charge storage region comprises a dielectricisolated floating gate, an ONO dielectric film or an insulating layercontaining conductive nanocrystals.
 4. The array of claim 3, furthercomprising: doped regions of a first conductivity type in the firstsemiconductor layer comprising portions of the source and drain regions;a first insulating layer located between the first rails; and a secondinsulating layer located between the second rail stacks.
 5. The array ofclaim 2, wherein the first rails are located below the second railstacks.
 6. The array of claim 2, wherein the first rails are locatedabove the second rail stacks.
 7. The array of claim 1, wherein thesecond set of TFTs comprises a set of totally disabled TFTs.
 8. Thearray of claim 1, wherein the second set of TFTs comprises a pluralityof totally disabled TFTs and a plurality of partially disabled TFTs. 9.The array of claim 1, wherein at least a portion of channel regions ofthe partially or totally disabled TFTs of the second set have beenselectively removed.
 10. The array of claim 1, wherein at least aportion of channel regions of the partially or totally disabled TFTs ofthe second set have been selectively doped to increase a thresholdvoltage of said TFTs to render said TFTs partially or totallyinoperative.
 11. A monolithic three dimensional TFT mask ROM array,comprising a plurality of device levels, each device level comprisingthe array of claim
 1. 12. A mask ROM array, comprising: a first set ofenabled transistors containing a charge storage region; and a second setof partially or totally disabled transistors.
 13. The array of claim 12,wherein the second set of transistors contain a charge storage region.14. The array of claim 12, wherein the transistors comprise TFTs. 15.The array of claim 14, wherein the second set of TFTs comprises a set oftotally disabled TFTs.
 16. The array of claim 14, wherein the second setof TFTs comprises a plurality of totally disabled TFTs and a pluralityof partially disabled TFTs.
 17. The array of claim 14, wherein at leasta portion of channel regions of the partially or totally disabled TFTsof the second set have been selectively removed.
 18. The array of claim14, wherein at least a portion of channel regions of the partially ortotally disabled TFTs of the second set have been selectively doped toincrease a threshold voltage of said TFTs to render said TFTs partiallyor totally inoperative.
 19. The array of claim 14, wherein: the TFTs areselected from one or more of top gate co-planar TFTs, top gate staggeredTFTs, bottom gate co-planar TFTs and bottom gate staggered TFTs; and thecharge storage region comprises a dielectric isolated floating gate, anONO dielectric film or an insulating layer containing conductivenanocrystals located between a gate and a channel of a TFT.
 20. Thearray of claim 19, further comprising: a first plurality of spaced apartconductors disposed at a first height above a substrate in a firstdirection; and a second plurality of spaced apart rail stacks disposedat a second height in a second direction different from the firstdirection, each rail stack including: a first semiconductor layer whosefirst surface is in contact with said first plurality of spaced apartconductors; a conductive film; and the charge storage region disposedbetween a second surface of the first semiconductor layer and theconductive film.
 21. The array of claim 20, wherein: the first pluralityof conductors comprise first rails comprising polysilicon layers incontact with metal or a metal silicide layers; the first semiconductorlayer comprises a polysilicon layer; the conductive film comprises apolysilicon layer and a metal silicide layer; and the TFTs are formed atintersections of two adjacent first rails and the second rail stack,such that the conductive film comprises a gate of the TFTs, portions ofthe first semiconductor layer comprise TFT channel regions and thepolysilicon layers of the first rails comprise at least a portion of theTFT source and drain regions.
 22. A monolithic three dimensional TFTmask ROM array, comprising a plurality of device levels, each devicelevel comprising the array of claim
 14. 23. The array of claim 12,wherein the array comprises an array of CMOS TFTs comprising the chargestorage region.
 24. The array of claim 23, wherein the programmed arraycomprises a static random access memory cell and a logic cell.
 25. Thearray of claim 24, wherein the static random access memory arraycomprises: a first set of enabled access and inverter TFTs; the secondset of disabled isolation TFTs having at least a portion of a channelregion removed; and a third set of isolation TFTs having at least aportion of a channel region removed and at least a portion of the chargestorage region removed such that a gate electrode contacts source ordrain regions of the TFTs of the third set.
 26. The array of claim 24,wherein the logic cell comprises at least one antifuse link connecting agate electrode and a source or a drain of a TFT.
 27. The array of claim12, comprising an EEPROM, the mask ROM and at least one antifuse linkconnecting a gate electrode and a source or a drain of a TFT.
 28. Thearray of claim 12, wherein at least one cell of the array is programmedby mask ROM programming techniques and at least one other cell isprogrammed by hot carrier injection or Fowler-Nordheim tunneling.